<body>
<input type="text" id="txt1">
<input type="button" id="btn" value="计算面积">
<input type="text" id="txt2" disabled>
</body>
<script>
var txt1 = document.getElementById("txt1");
var txt2 = document.getElementById("txt2");
var btn = document.getElementById("btn");
btn.onclick = function(){
var s = mianji(txt1.value);
txt2.value=s;
}
function mianji(a) {
var mj = 3.14 * a * a;
return mj;
}
</script>
